The art of duplexing, or printing on both sides of a sheet of paper (or other media), has long been reserved for higher end imaging devices, including inkjet printers, laser printers, photocopiers, etc. The complexities associated with duplex imaging are well known in the art. For example, U.S. Pat. No. 4,453,841 describes a Duplex Printing system And Method Therefor wherein multiple copy sheets are transported simultaneously through a sheet path. A copy sheet inverter receives the copy sheets sequentially from the sheet path after the sheets are printed on their first sides and returns them to the sheet path in an inverted orientation prior to printing on the second sides.
A separate implementation for duplexing is noted in U.S. Pat. No. 4,066,252 wherein after a sheet is printed on a first side, it is transported to an output accumulation tray, but then withdrawn therefrom and re-introduced into the paper path while at the same time being inverted for imaging on the second side.
Duplexing systems such as that described in the '841 patent are generally more complex, costly and faster than systems such as that described in the '252 patent. As such, duplexing systems that incorporate the technique of withdrawing the sheet from the output tray to return it for duplexing (similar to the description of the '252 patent) are generally implemented in lower end, lower cost systems, with the sacrifice of speed being exchanged for reduced cost. However, in either type of system, there is a continual effort to reduce costs, increase speed, and provide improved options for the customer.
To this end, it is also known in the art to implement duplexing systems by creating the duplexing feature as an add-on option to an already existing imaging system, such as a laser printer. This enables a lower cost non-duplexing system that can be subsequently upgraded to a duplexing system by the purchase of the add-on duplexer. However, these add-on type duplexers generally require separate drive motors and duplicate components relative to what already exist in the imaging system to which the add-on duplexer is added. Thus, overall costs may actually be greater than if the duplexer is purchased as an integrated feature of the original imaging system.
Accordingly, an object of the present invention is to enable a low cost duplexer through the integration of certain duplexing functionality into the base engine while retaining some add-on type capability.